Techniques for efficient data transfers in a body area network

ABSTRACT

A method for transferring data among devices in a body area network (BAN). The method comprises dividing an access time to a wireless medium of the BAN into at least a contention-based period and a contention-free reservation period; allowing devices to transfer data during the contention-based period using a local prioritized contention access (LPCA) mechanism; and allowing only devices having reserved time slots to transfer data during the contention-free reservation period.

The invention generally relates to medium access control (MAC) protocolsutilized in low power wireless sensor networks, such as body areanetworks (BANs).

A body area network (BAN) is primarily designed for permanent monitoringand logging of vital signs. An exemplary BAN 100, shown in FIG. 1,includes multiple nodes 120 which are typically sensors that can beeither wearable or implantable into the human body. The nodes 120monitor vital body parameters and movements, and communicate with eachother over a wireless medium. The nodes 120 can transmit data from abody to one or more devices 130 from where the data can be forwarded, inreal-time, to a hospital, clinic or elsewhere over a local area network(LAN), a wide area network (WAN), a cellular network, and the like.

The requirements for designing BANs include energy efficiency of nodes120, scalability, integration, interference mitigation, coexistence,high quality of service (QoS), and security. Efficient energyconsumption can be achieved by optimally duty cycling a receiver device(i.e., a device receiving data) between a listen state and a sleepstate. In the sleep state a radio transceiver of the device is turnedoff, thereby saving energy. A duty cycling is performed by a MACprotocol with the aim of minimizing idle listening, overhearing,collisions and controlling overhead.

The IEEE 802 standards committee has developed a family of standards forwireless local and personal area networks, such as the IEEE 802.11designed for wireless local area networks, and the IEEE 802.15.4designed for wireless personal area networks (WPANs). None of theseprotocols is a suitable candidate for wireless BANs. For instance, theIEEE 802.15.4 standard defines a MAC protocol for short rangetransmissions which suffers from several limitations preventing thisprotocol from being utilized in BANs.

Specifically, the IEEE 802.15.4 standard beaconing mode supports starand tree network topologies. The network coordinator establishes thenetwork and becomes the root of the tree. The nodes in the tree haveparent-child relationships. Typically, the data is transferred between aparent and a child. The leaf nodes in the tree typically do not sendbeacons. Such a transfer mode is not suitable for BAN applications dueto the risk of a single point of failure.

The active period of beaconing devices is fixed a priori and is the samefor all the beaconing devices belonging to a network. This could resultin either over provisioning and wasted energy or under provisioning andlimited QoS. Since the duty cycle requirement of BAN devices varies fromdevice to device and from time to time, the fixed duty cycling approachof IEEE 802.15.4 is not suitable for the BAN.

In addition, in the IEEE 802.15.4 beacon-enabled mode, the networkcoordinator can optionally allocate a limited number (e.g., 7) ofguaranteed time slots which are typically not sufficient to provide thedesired level of QoS for BAN applications.

For at least the shortcomings described above, it would be thereforeadvantageous to provide a solution for reserving time slots for datatransfer and for transferring data between devices to support thedifferent requirements of BAN applications.

Certain embodiments of the invention include a method for transferringdata among devices in a body area network (BAN). The method comprisesdividing an access time to a wireless medium of the BAN into at least acontention-based period and a contention-base reservation period;allowing devices to transfer data during the contention-based periodusing a local prioritized contention access (LPCA) mechanism; andallowing only devices having reserved time slots to transfer data duringthe contention-free reservation period.

Certain embodiments of the invention further include scheduling timeslots reservation requests for master devices in a body area network(BAN). The method comprises receiving, during a current time round, fromeach master device a reservation request including at least a number oftime slots to be allocated in a superframe in a next time round, whereinthe reservation request is through a global beacon; and sequentiallyscheduling the requests for reservation by contiguously allocating timeslots in each superframe.

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe invention will be apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram of a body area network.

FIG. 2 illustrates a topology of a body area network utilized todescribe the various embodiments of the invention.

FIG. 3 is a two-dimensional representation of a time round.

FIG. 4 is a two-dimensional organization of a time round derived bysequentially scheduling reservation requests.

FIG. 5 is a diagram for illustrating an allocation of time slots in asuperframe in accordance with an embodiment of the invention.

FIGS. 6A and 6B are diagrams for illustrating contention-based andreservation based data transfers between master devices implemented inaccordance with an embodiment of the invention.

FIGS. 7A and 7B are diagrams for illustrating contention-based andreservation based data transfers from a master device to a slave deviceimplemented in accordance with an embodiment of the invention.

FIGS. 8A and 8B are diagrams for illustrating contention-based andreservation based data transfers from a slave device to a master deviceimplemented in accordance with an embodiment of the invention.

FIGS. 9A and 9B are diagrams for illustrating contention-based andreservation based data transfers between slave devices implemented inaccordance with an embodiment of the invention.

It is important to note that the embodiments disclosed by the inventionare only examples of the many advantageous uses of the innovativeteachings herein. In general, statements made in the specification ofthe present application do not necessarily limit any of the variousclaimed inventions. Moreover, some statements may apply to someinventive features but not to others. In general, unless otherwiseindicated, singular elements may be in plural and vice versa with noloss of generality. In the drawings, like numerals refer to like partsthrough several views.

FIG. 2 shows a topology of a body area network (BAN) 200 utilized todescribe the various embodiments of the invention. The BAN 200 includestwo tiers of devices: slave devices 210-1 through 210-S and masterdevices 220-1 through 220-M. Typically, the slave devices 210-1 to 210-Sare implantable, swallowable or disposable and characterized by havinglow energy budgets and limited resources (e.g., processing power,memory). On the other hand, the master devices 220-1 to 220-M arewearable, can be recharged frequently and therefore have higher energybudgets and more resources than the slave devices.

A master device 220-Z (where Z is an integer equal to or greater than 1)manages one or more slave devices 210-G (where G is an integer equal toor greater than 1). To this end, a master device 220-Z transmitsperiodic beacons for synchronization, requesting medium reservation, andannouncing broadcast/multicast. Based on the information exchanged bythe periodic beacons, the master device 220-Z derives a conflict-freereservation schedule to enable QoS support. In addition, the masterdevice 220-Z detects the presence of another BAN located within itstransmission range to support the harmonized coexistence of multipleco-located BANs, each of which can potentially execute a differentapplication.

In the topology illustrated in FIG. 2, all master devices 220-1 to 220-Msynchronize the medium access and implement reservation using adistributed global beaconing process. In a preferred embodiment of theinvention the access to the medium is divided into fixed and repeatedduration time rounds, where a time round is a data structure designed toinclude a predefined number of superframes, each of which includes afixed number of time slots.

FIG. 3 shows an exemplary and non-limiting two-dimensionalrepresentation of a time round 300. The X-axis represents the time slotsper superframe, and the Y-axis represents the available superframes pertime round 300. A superframe is a data structure utilized to exchangeinformation between master devices and between a master device and itsrespective slave devices.

Master devices can reserve time slots during which the devices have anexclusive right to access the medium. Predefined time slots in a timeround are reserved for a global beacon period (GBP) 310. Such time slotsare utilized for transmitting global beacons required to facilitateperiodic synchronization of master devices. The master devices listen tothe global beacon period 310 and send global beacons in their allocatedtime slots to synchronize and exchange medium reservation requests.Global beacons are also used to discover neighbors and network topology,provide QoS, and schedule the broadcasting or multicasting of messages.

In accordance with an embodiment of the invention global beacons aretransmitted using a scheduling method that dynamically constructs andmaintains a logical tree topology of master devices in the BAN.Accordingly, a global beacon period is divided into two time periods: anascending period (AP) and a descending period (DP).

During the AP, all master devices, but the root device, transmit theirglobal beacons in their respective slots in ascending order, i.e.,children transmit their global beacons before their parents. During theAP, parents listen to their children's global beacons. During the DP thesequence of global beacon transmissions is reversed, i.e., parentstransmit their global beacons before their children. In this periodchildren listen to their parent's global beacons.

During the AP, global information is passed from children to ancestors(parents). At the end of the AP, the root device knows the completeglobal information which it distributes to all the master devices duringthe DP. Thus, the global beacon scheduling method ensures that all themaster devices belonging to the same BAN receive the global informationeven though they are hidden (not in direct communication range) fromeach other.

At the end of a global beacon period 310, all master devices know thereservation requests from every other master device. In one embodimentof the invention only revised reservation requests are propagated toother master devices. In the absence of revised reservation requests,the requests of the last round are preserved.

In accordance with certain principles of the invention the globalinformation about reservation requests is used to derive a unique,consistent and non-overlapping schedule of subframe transmissions. Asubframe represents a contiguous block of slots allocated to a masterdevice. With this aim, each master device independently schedulessubframes, but arrives at a unique and consistent schedule ofconflict-free reservations. Thus, all the master devices know thelocations of their peers' devices subframes in the next round.

In one embodiment of the invention a scheduling method executed by amaster device includes scheduling all the requests sequentially (e.g.,based on MAC addresses or on a first come first serve basis) providedthat sufficient time slots are available. When sufficient slots are notavailable to accommodate all the reservation requests, requests can beprioritized based on service categories. Gathering reservation requestsand scheduling of the subframes for a next time round is performed in acurrent time round.

As a non-limiting example, FIG. 4 shows a two-dimensional organizationof a time round 400 derived by sequentially scheduling reservationrequests. The time round 400 consists of 4 superframes 410, andreservation requests received from master devices are as listed inTable 1. As depicted in FIG. 4, time slots 420 in a superframe 410 arecontiguously reserved according to a number of time slots requested byeach master device.

TABLE 1 Superframe Masters' request for slots ID A B C D 0 6 3 0 0 1 0 05 3 2 4 5 0 0 3 0 0 8 0

The global knowledge of reservation requests results in compactscheduling where time slots are allocated contiguously. The unreservedtime slots are grouped together towards the end of a superframe and canbe used as, for example, global prioritized contention access (GPCA) tothe medium. This results in efficient channel utilization. Furthermore,the contiguous time slots allocation results in fewer transitions from asleep mode to an active mode which improves the energy efficiency.

In accordance with another embodiment of the invention an advancedscheduling method can be utilized to allocate subframes based on thepriority levels of underlying applications and services. For example,medical applications should not be deprived of medium access due toexisting reservations by entertainment applications. The globalknowledge of reservation requests can help devices to exercise effectiveadmission control and prioritization.

In accordance with an embodiment of the invention, a master device canbroadcast or multicast messages to other master devices (referred to asglobal broadcast/multicast). To this end, master devices embed a requestfor global broadcast/multicast in their global beacons. The schedulingmethod determines time slots reserved for global broadcast/multicastdata transfers which are known to all master devices. Intended masterdevices listen to the slots reserved for global broadcast/multicast. Amaster device that receives a broadcast/multicast message determines ifthe messages need to be forwarded to its slave devices.

It should be appreciated that the subframe scheduling methods describedherein enable dynamic and scalable duty cycling that can be adaptedaccording to latency requirements and traffic conditions of the BAN.These techniques thereby allow for saving power without compromisingQoS.

Certain embodiments of the invention also include a method for enablingefficient data transfers between peer master devices and between amaster device and its respective slave devices. The method employsprioritized contention-based access for differentiated QoS classes andreservation based access for reliable communications between devices.

As mentioned above, master devices reserve time slots for theirrespective subframes using global beacons, and timings of subframes areknown globally to all the master devices.

Time slots within a subframe can be reserved for QoS enabled or periodictraffic, while unreserved time slots within a subframe can be accessedusing local prioritized contention access (LPCA) mechanism for on-demandtraffic. This is further illustrated in FIG. 5, where a subframe 500includes contention-based access (LPCA) period 510 and contention-free520 (reservation based) access period.

The reservation time must occupy contiguous time slots. Thecontention-free period 520 grows or shrinks depending on the totallength of combined reserved time slots. Each device transmitting inreserved time slots ensures that its transaction (DATA andacknowledgment) is completed before the reserved time expires.

In one embodiment, the reserved slots are de-allocated when they are nolonger required. The reservation owner or the master device cande-allocate slots at any time at its discretion, for example, if a timeslot is unused for a predefined duration. A device that has beenallocated time slots may also operate in the LPCA period.

Each reserved time slot can be marked as transmit or receive relative tothe data flow from the device that owns the reservation. For eachallocated reservation the owner device stores the type of reservation,its starting time slot, length, number of slots, direction, periodicityand associated device address.

The contention-based access period 510 is used for on-demand access, aswell as for new devices joining the BAN. All the frames, exceptacknowledgement frames and any data frames that follow the requestframes, transmitted during the contention-based access period 510utilize the LPCA mechanism to access the medium.

FIG. 6A illustrates the contention-based data transfer from a masterdevice 220-A to a master device 220-B implemented in accordance with anembodiment of the invention. Master devices 220-A and 220-B know thereservation schedule of each other. At T1, device 220-A wakes up andlistens to a local beacon of device 220-B to synchronize and locate acontention-based (LPCA) period 610. A local beacon is part of a subframeand typically sent by a master device to its peer master devices orslave devices. Typically, local beacons carry synchronization, time slotallocation and boundary information, which help slave devicessynchronize with their master devices, find allocated slots and deducethe boundary of the LPCA period. Device 220-A contends for the medium totransmit data to device 220-B during the LPCA period 610 in one of thesubframes of device 220-B. Depending on the acknowledgement policy,device 220-B may acknowledge successful reception of the data fromdevice 220-A.

If device 220-A has more data for device 220-B which it may not be ableto transmit to device 220-B during a current subframe, then device 220-Acan either wait until the next subframe of device 220-B arrives (if datais not time sensitive) or device 220-A can request device 220-B toremain awake during a designated GPCA period. The devices 220-A and220-B can communicate during the GPCA period if both agree to remainawake during this period. This can be performed if one or more of thefollowing conditions apply: data to be transferred by device 220-A istime sensitive; device 220-A buffers are nearly full; subframesallocated to device 220-B are widely spaced; and device 220-Aexperienced significant contention and back offs while trying to accessthe medium during a subframe allocated to device 220-B.

FIG. 6B illustrates the reservation based data transfer from a masterdevice 220-A to a master device 220-B implemented in accordance with anembodiment of the invention. At T1, device 220-A wakes up and listens toa local beacon of device 220-B to synchronize and locate thecontention-based (LPCA) period 610. Device 220-A contends for the mediumto transmit a reservation request during the time period 610. Thereservation request identifies a number of time slots, direction of datatransfer, and ownership information. Device 220-B may acknowledge therequest. If the request can be partially or fully accommodated in thesubframes allocated to device 220-B during a current time round, slotsare allocated in those subframes. Otherwise, device 220-B updates itsreservation requests for the next time round and secures the time slotsfor device 220-A during its subframes in the subsequent time rounds.Device 220-B may secure new subframes at desired locations in a timeround via global beacons to accommodate device 220-A request.

The time slot allocation information is embedded in a local beacontransmitted by device 220-B. At time T2, device 220-A listens to a localbeacon of device 220-B to locate the slots allocated to it.Subsequently, device 220-A transmits to device 220-B during the timeslots 620 allocated to 220-A by 220-B. If time slot allocationinformation is not found in the local beacon within a predefinedtimeframe, device 220-A will notify a failure to a higher layer.

FIG. 7A illustrates the contention-based data transfer from a masterdevice 220-A to a slave device 210-X implemented in accordance with anembodiment of the invention. At T1, slave device 210-X listens to alocal beacon of device 220-A to synchronize and locate thecontention-based (LPCA) period 710. The local beacon further indicatespending messages (if any) to the slave device 210-X. Upon reception ofthe local beacon, slave device 210-X transmits a request to receive thepending messages to device 220-A during the LPCA period 710 of asubframe allocated to master device 220-A. Depending on theacknowledgement policy, master device 220-A may acknowledge the requestsent from slave device 210-X. Thereafter, master device 220-A transfersthe pending messages to the slave device 210-X. The devices 220-A and210-X may decide to continue the message exchange during a designatedGPCA period. Upon successful completion of the data transmission, themessage is removed from a list of pending messages in the local beacon.

FIG. 7B illustrates the reservation based data transfer from masterdevice 220-A to slave device 210-X implemented in accordance with anembodiment of the invention. Master device 220-A indicates in its localbeacon its intention to reserve time slots for slave device 210-X.Master device 220-A also indicates the reservation repetition period. AtT1, slave device 210-X wakes up, listens to a local beacon of masterdevice 220-A and learns about pending reservation requests. Slave device210-X acknowledges the reservation during a LPCA period 710. Uponreceiving the acknowledgement, master device 220-A allocates time slotsfor slave device 210-X.

After the reservation is established, slave device 210-X tracks localbeacons of master device 220-A to synchronize and locate the slotsallocated to it. Device 220-A transmits data to device 210-X during thetime slots 720 reserved for slave device 210-X. Depending on theacknowledgement policy, slave device 210-X may acknowledge the messagessent from device 220-A. Devices 220-A and 210-X may decide to continuethe message exchange during a designated LPCA or GPCA period.

FIG. 8A illustrates the contention-based data transfer from a slavedevice 210-X to a master device 220-A implemented in accordance with anembodiment of the invention. At T1, slave device 210-X listens to alocal beacon sent by master device 220-A to synchronize and locate acontention-based (LPCA) period 810. Thereafter, slave device 210-Xtransmits to master device 220-A during the LPCA period 810. Dependingon the acknowledgement policy, master device 220-A may acknowledge themessages from slave device 210-X. Devices 220-A and 210-X may decide tocontinue the message exchange during a designated GPCA period.

FIG. 8B illustrates the reservation based data transfer from a slavedevice 210-X to a master device 220-A implemented in accordance with anembodiment of the invention. At T1, slave device 210-X listens to alocal beacon of master device 220-A to synchronize and locate acontention-based (LPCA) period 810. During this period, slave device210-X transmits a reservation request to master device 220-A. Device220-A may acknowledge the request. If the request can be partially orfully accommodated in the subframes allocated to device 220-A during thecurrent round, then time slots are allocated in those subframes.Otherwise, master device 220-A updates its reservation requests for thenext round and secures the time slots 820 for slave device 210-X duringthe subframes allocated to master device 220-A in subsequent timerounds. It should be noted that master device 220-A may secure newsubframes at desired locations in a time round via a global beaconexchange to accommodate requests sent by slave device 210-X.

The slot allocation information is embedded in local beacons transmittedby master device 220-A. At T2, slave device 210-X listens to a localbeacon of master device 220-A to locate the time slots 820 allocated toit. Device 210-X can transmit during its reserved time slots 820. Inaddition, devices 220-A and 210-X may decide to continue the messageexchange during a designated LPCA or GPCA period. If slot allocationinformation is not found in the local beacon within a predefinedtimeframe, slave device 210-X will indicate a failure to a higher layer.

FIG. 9A illustrates the contention-based data transfer from a slavedevice 210-Y to a slave device 210-X implemented in accordance with anembodiment of the invention.

The slave devices can sleep for a prolonged duration of time: a slavedevice does not know when other slave devices are scheduled to wake-up.Therefore, slave device 210-Y does not know in which subframe device210-X wakes up. Moreover, when slave device 210-X wakes up, it listensonly to local beacons of a master device 220-A to check if there is anypending message for it. If there are no pending messages, the device210-X immediately returns to a sleep state. Therefore, devices 210-X and210-Y cannot directly communicate with each other even though they areneighbors.

Data transfer between slave devices is performed through a masterdevice. Specifically, at T1, device 210-Y listens to a master device220-A local beacon to synchronize and locate a contention-based (LPCA)period 910. Then, device 210-Y transmits the message to a master device220-A during the LPCA period 910 of a subframe of master device 220-A.Thereafter, master device 220-A forwards the message to slave device210-X as explained above. Devices 210-X and 210-Y may decide to continuethe message exchange during a designated LPCA period.

FIG. 9B illustrates the reservation based data transfer from a slavedevice 210-Y to a slave device 210-X implemented in accordance with anembodiment of the invention. At T1, slave device 210-Y sends areservation request to slave device 210-X via a master device 220-A, asdescribed above with reference to FIG. 8B. At T2, slave device 210-Xlistens to a local beacon sent by the master device 220-A that includesat least the information about time slots to be reserved for datatransfers from slave device 210-Y to slave device 210-X. At T3, slavedevice 210-X acknowledges the request, and master device 220-A allocatesthe time slots to be reserved for the data transfers. If the request canbe partially or fully accommodated in the subframes allocated to masterdevice 220-A during the current round, the time slots are allocated inthose subframes. Otherwise, master device 220-A updates its reservationrequests for the next round and reserves time slots for slave devices210-X and 210-Y during subframes in the subsequent rounds. The timeslots allocation information is embedded into local beacons sent bymaster device 220-A. At T4, slave devices 210-X and 210-Y listen to alocal beacon of master device 220-A to locate the time slots allocatedto devices 210-X and 210-Y. Devices 210-X and 210-Y can communicateduring the reserved slots 920. Devices 210-X and 210-Y may also decideto continue the message exchange during a designated LPCA or GPCAperiod. If time slot allocation information is not found in the localbeacon within a predefined timeframe, devices 210-X and 210-Y report afailure to a higher layer.

In accordance with another embodiment of the invention, a master devicecan broadcast or multicast messages to its respective slave devices(local broadcast/multicast). To this end, the master device reserves thetime slot for local broadcast/multicast of messages in its subframes andannounces the reservation in its local beacons. Slave devices listen tolocal beacons and learn about the scheduled broadcast/multicast and thecorresponding reserved time slots. Intended slave devices listen to thedesignated time slots to receive the broadcast/multicast transmission.Master devices may have to store broadcast messages for sleeping slavedevices. Slave devices can broadcast/multicast messages through theirrespective master devices.

The foregoing detailed description has set forth a few of the many formsthat the invention can take. It is intended that the foregoing detaileddescription be understood as an illustration of selected forms that theinvention can take and not as a limitation to the definition of theinvention. It is only the claims, including all equivalents that areintended to define the scope of this invention.

Most preferably, the principles of the invention are implemented as anycombination of hardware, firmware and software. Moreover, the softwareis preferably implemented as an application program tangibly embodied ona program storage unit or computer readable medium. The applicationprogram may be uploaded to, and executed by, a machine comprising anysuitable architecture. Preferably, the machine is implemented on acomputer platform having hardware such as one or more central processingunits (“CPUs”), a memory, and input/output interfaces. The computerplatform may also include an operating system and microinstruction code.The various processes and functions described herein may be either partof the microinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU, whether or not suchcomputer or processor is explicitly shown. In addition, various otherperipheral units may be connected to the computer platform such as anadditional data storage unit and a printing unit.

1-13. (canceled)
 14. A method for scheduling time slots reservationrequests for master devices in a body area network (BAN) including aplurality of slave devices and a plurality of master devices arranged ina two-tier architecture, comprising: receiving, at the plurality ofmaster devices, during a current time round, from each device requestingto send data, a reservation request including at least a number of timeslots to be allocated in a superframe in a next time round, wherein thereservation request is through a global beacon; and sequentiallyscheduling the requests for reservation by contiguously allocating timeslots in each superframe to at least one peer slave device of at leastone master device of the plurality of master devices.
 15. The method ofclaim 14, wherein time slots are reserved for data transfers betweenmaster devices, between a master device and its respective peer slavedevices, and between slave devices.